Incremental optical motion encoders are used to resolve the position and movement of an object along a particular route. Such encoders generally include a light source for emitting a light beam, light modulation means for modulating the light beam in response to movement of the object along the route, and a detector assembly for receiving the modulated light and for producing electrical signals indicating the amount of light received by the detectors. As the light is modulated in response to movement of the object, each electrical signal from the detector assembly produces a waveform. The position of the object along its route determines the position of each signal on its particular waveform, that is, the phase of each signal. Thus, the electrical signals from the detectors can be used to indicate the change in location of the object along the route. Two or more properly out-of-phase signals from separate detectors can be used to indicate both change in location and direction of movement.
In order for an incremental motion encoder to produce an indication of the absolute position or location of the object along its route, an index pulse is required at least once along the route. The incremental signals can be used to count incremental movement from the index pulse, and if the position of the object is known at the index pulse, the absolute position of the object at any place along the route can be determined. Thus, to provide an indication of absolute position, change in location, and direction of movement, an incremental encoder generally requires three channels of information, two channels derived from two or more out-of-phase encoder signals that are produced throughout the route of the object and an index signal that is produced at least once along the route and at a known position of the object.
Index or reference signals may also be used in incremental motion encoders for purposes unrelated to providing absolute position information and may also be used in absolute position encoders. For example, a reference pulse may be used to detect errors in the other encoder signals by counting encoder signals between reference pulses. Excess or insufficient counts between reference pulses indicate an error in the encoder signal. However, regardless of wheter an index or reference signal is used in deriving absolute position information or in encoder error detection, the timing or phase relationship between the index or reference signals and the other encoder signals is often critical to proper operation of the encoder.
As shown in FIG. 1, the prior art optical shaft encoders may modulate a light beam with two separate elements, a code wheel that is mounted on the shaft whose movement is being encoded, and a stationary phase plate. The modulated light beam is converted into two analog electrical signals by two separate incremental movement light detectors. From these analog signals, two out-of-phase pulse trains can be produced, and from these pulse trains, change in rotary position and direction of shaft rotation can be determined.
A reference or index pulse may be produced with a push-pull system in which light is modulated with two separate light modulation index tracks on the rotating code wheel and the stationary phase plate. The light modulated by the two index tracks is received by separate index light detectors, and the detectors produce two separate index analog signals from which an index pulse may be derived. The index pulse may be produced by a comparator that compares the two index signals and changes logic states when the levels of the two signals are equal; that is, the index pulse may be a logic state signal between the crossover points of the two analog index signal waveforms. In the illustrated prior art encoder, the index pulse occurs once each revolution of the shaft and attached code wheel.
Prior art optical shaft encoders, such as the type depicted in FIG. 1, were sensitive to misalignment, particularly misalignment between the code wheel and the phase plate. Due to their sensitivity to misalignment, the fabrication and application of these prior art encoders required elaborate alignment procedures and equipment, and generally raised the costs of the encoders.
Phase errors in the incremental movement signals, that is, timing errors between the signal waveforms, were introduced by misalignments in the radial direction, shown in FIG. 1 by arrow R. Also, the timing or phase of the index pulse with respect to the incremental movement signals was sensitive to misalignments in the tangential direction, shown by arrow T in FIG. 1. Phase shifts between the incremental movement signals and index signals due to misalignment in the tangential direction arise because the tracks are at different radii on the code wheel. Where the code wheel is in a tangentially misaligned position, a point on the incremental movement track may be approximately aligned with a corresponding point on the phase plate or detector assembly by rotating the code wheel. However, the amount of rotation needed depends not only upon the magnitude of the tangential misalignment but also upon the radius of the incremental movement track, the required rotation increasing as the radius decreases. For example, the two relatively small radius index tracks shown in FIG. 1 require a greater rotation to correct for tangential misalignment than is required for the incremental movement track having a larger radius. The difference in rotation angles required to achieve compensation for tangential misalignment translates into a phase error between the signals from two radially separated tracks, such as the two radially separated index tracks in FIG. 1.
The prior art optical shaft encoder shown in FIG. 2, and described in U.S. Pat. No. 4,691,101, eliminates the requirement of a phase plate, and uses only a code wheel to modulate the optical beam. To provide the required electrical signals, this particular type of optical encoder uses an array of side-by-side incremental movement light detectors. Each detector in the array produces an analog output signal in response to light modulated as the code wheel rotates, and these analog output signals can be processed to produce two out-of-phase square-wave pulses from which shaft rotation and direction of rotation can be determined.
The prior art encoder shown in FIG. 2 is substantially less sensitive to misalignment than the code wheel/phase plate encoder shown in FIG. 1. This detector array-type prior art encoder, however, does not produce an index pulse for providing absolute positional information. If absolute position information was required, a push-pull system similar to that shown in FIG. 1 could be used. However, such an index pulse would still have been very sensitive to tangential misalignment between the code wheel and the detectors. Thus, the use of an index pulse would have substantially defeated the misalignment insensitivity benefits gained by the detector array-type prior art encoder shown in FIG. 2.
Although optical shaft rotation encoders are shown in FIGS. 1 and 2 for purposes of illustration and ease of description, misalignment problems are by no means limited to shaft rotation encoders. Similar alignment problems with regard to the index modulation tracks are encountered with encoders that indicate the movement of an object along a linear or other route. Index or reference signals in linear motion encoders are sensitive to rotational misalignment between the linear modulator tracks and the detectors. Also, although optical encoders are used to illustrate the misalignment problem with differential signal encoders, other types of electromagnetic radiation may be used if suitable emitter, detector, and modulation means are available.
It is therefore an object of the invention to provide an incremental movement optical encoder that can provide absolute position information as well as direction of movement information, and that is relatively insensitive to misalignments encountered in fabrication and in application to a particular use.
Another object of the invention is to provide a detector array-type encoder for indicating an object's movement along a route that produces an index or reference signal that is insensitive to misalignments encountered in fabrication and application of the encoder.
A further object of the invention is to provide a modulation member adapted for use in a motion encoder for producing a phase insensitive index or reference signal.